Various types of fillers may be used in polymeric materials to affect their material properties. For example, certain non-organic fillers, such as glass fibers or other mineral-based fillers, may be used to increase the rigidity or modulus of a polymer matrix to make it more useful as a structural material or to increase its resistance to deformation when exposed to high temperatures or loads. Other types of fillers, such as elastomeric materials, are sometimes used to increase the toughness or impact strength of a polymer matrix. Such fillers are typically included in amounts that are sufficient to impart one or more properties of the filler material at least partially to the resulting composite material. For example, glass fibers may be added to a polymer matrix to make a composite material with a tensile modulus somewhere between the tensile modulus of the polymer matrix and the glass fibers. It is not unusual for such polymer composites to include fillers in amounts of 25% or more.
In certain applications, attempts are made to balance filler types and amounts to provide an optimum combination of mechanical properties. For example, a polymer-based material for use in hinge-type airbag covers is described in U.S. Pat. No. 6,644,684 to Tsuji et al. The Tsuji patent discloses certain thermoplastic elastomer (TPE) material formulations in an attempt to balance high material rigidity with high toughness; the high rigidity being desired for resistance to heat deformation and deformation due to passenger contact with the cover, and the high toughness being desired so that the cover does not break into loose pieces that get projected into the vehicle interior upon airbag deployment. However, some materials that toughen polymers also reduce their rigidity. Tsuji purports to solve this problem by providing an olefin-based plastic material that is modified with elastomeric materials having particular chemical compositions in specific amounts, teaching that such materials may have sufficient stiffness to avoid deformation in use while having sufficient toughness to avoid breakage of the material when the hinge-type airbag cover opens during airbag deployment.